Because some other people you want to communicate with have other phones. They don't do email, don't do IM, don't have a data plan, live in the middle of nowhere, don't even have internet at home, can't read text on a phone screen but can see a picture, etc.

Seriously, how else are you going to send a picture to grandma who is dirt poor and has a basic cheap phone on the basic $10/month package with insert-operator-with-no-3G-coverage-here? (Bear in mind MMS are free to recieve in many countries).

I honestly don't understand how anyone could not get this. Do you only communicate with people like yourself? Do you assume that because you have an email-capable, IM-capable phone that magically everyone you communicate with does too? Or do you discard friends who don't because they're not modern enough for you? Perhaps you just never talk to such people in the first place?

Or maybe you're real generous and you buy everyone you know an N900 and put their data bill on your credit card?